For each \( 0 \leqslant \theta \leqslant 1 \) , let \( [X^*,Y]_\theta \)  and \([X,Y]_\theta \)  be the interpolation spaces defined by the famous complex
interpolation method (cf. [1]}). We have that \([X^*,Y]_\theta \)  is the dual space of \( [X,Y]_\theta \)  and vice versa
(cf. [2]}). By (REF ), (REF ) and [1]}, a straightforward computation gives that
\(\Vert {\mathcal {A}} \Vert _{\mathcal {L}( [X,Y]_\theta , [X^*,Y]_{1-\theta } )} \leqslant c_1 \quad \text{for all } 0 \leqslant \theta \leqslant 1.\) 
